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scale their environments.

As the number of virtual desktops per host ramps up, the aggregated and seemingly random I/O from those desktops stress many parts of the I/O path, and any latency introduced by such issues impacts end-user performance and productivity -- not to mention acceptance of virtual desktops as an alternative to PCs, said Mike Matchett, a consultant at the Taneja Group, a technology analysis firm based in Hopkinton, Mass.

Washington Trust Bank, a large commercial bank based in the Northwest, ran into a storage problem when it moved to VMware View and Windows 7. It needed to provide persistent virtual desktops to workers who required a wide range of applications, said Christopher Green, vice president of IT infrastructure systems for the bank.

"When we calculated the ROI, we factored in that we would be able to use our existing resources," Green said. "We were only running 20% to 30% capacity and thought we would have enough."

Green's team had read through EMC Corp. and VMware Inc.'s reference architectures and expected the View launch to go smoothly.

"We used our existing [storage area network] and had plenty of disk capacity, but underestimated the impact that VDI and Windows 7 would have on the storage processes, due to the IOPS required," Green said. "We targeted 450 users and only got to 90 when we got helpdesk calls about performance issues."

Server RAM for VDI storage issues

Atlantis Computing's ILIO Persistent VDI 4.0, announced in February, gives Citrix XenDesktop and VMware View customers a way to run persistent virtual desktops using RAM as primary storage.

It basically creates a local RAM disk on each VDI host and redirects all I/O within each virtual desktop to the RAM disk while de-duplicating across all the desktops to optimize RAM disk capacity, Matchett said.

"You really can't get faster I/O than servicing it from local on-server memory, so I/O performance is going to be better than an added flash solution, SSD or cache," he said.

Atlantis' previous ILIO software was only for non-persistent VDI scenarios. With this release, it has added a replication host that on the back side of the RAM disk replicates all write I/O, and marshals it for efficient persistence in external storage (storage area network or network-attached storage), Matchett explained.

Atlantis said that ILIO provides a user experience above that of a PC at an infrastructure cost that is less than $300 per desktop.

In reality, the "better than PC experience" VDI vendors strive for depends on more than I/O performance, network latency and cluster balancing. But the Atlantis ILIO approach realistically promises higher performance at a lower cost/desktop than other VDI storage options, Matchett said. Other industry watchers, such as Brian Madden, agree.

Washington Trust Bank considered some Hybrid Flash-Solid State Disk (SSD) options, but determined the Capex would be $200 per user in addition to what the bank had already paid per user for VDI -- adding over $100,000 to the project cost, Green said.

They were able to get 450 users on the existing storage system with capacity to spare, Green said.

Atlantis' product requires at least 500 MBs of server RAM per user and storage capacity in the range of 30 GB to 80 GB per persistent desktop. The bank didn't have to increase server RAM because it had over spec'd to leave room for growth, Green said.

"You might need additional RAM, but even if you have to buy more RAM, with what it costs for the Atlantis product, you still only pay roughly half what you would spend on an extra SAN," Green said.

What do you think of using server RAM?

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